PARIS — The authorities in Belgium have warned security services that the Islamic State might be planning an imminent attack, with a group of extremists having departed Syria and divided into groups, headed for Belgium and France.

The warning was reported on Wednesday by the Belgian newspaper La Dernière Heure, which said the police and other security officials had received an alert saying that “combatants left Syria about a week and a half ago in order to reach Europe via Turkey and Greece, by boat, without passports.”